Small aircraft pilot reported a loss of proper terrain clearance while being vectored for landing at JYO due to ATC airspace constraints.

Narrative

Eastbound/westerly arrivals into JYO; encountered terrain/obstacle issues due to airspace constraints associated with the IAD Class B airspace design. Specifically; the JYO Airport; a significantly busy secondary airport in the Class B footprint; has a Control Tower without associated Class D airspace; possibly because of the virtual Control Tower; meaning that it does not have the normal control volume normally associated with a Class D airport.Because the JYO Tower does not control the airspace immediately above the airport; VFR arrivals from the west must simultaneously arrive below the 1500 ft. MSL Class B shelf at the AML 12 DME arc and clear terrain rising to an 835 point elevation at the AML 320/12.2 DME. During my arrival today; Tower directed arrival at or below 1400 ft. MSL even when I requested a higher altitude because of the terrain; and Tower further directed me onto a downwind leg at the AML 11.7 DME while simultaneously calling out an interval traffic on final to Runway 17 at JYO. Thus; I took a Tower-directed turn into rising terrain; toward 1087 ft. and 1338 ft. point elevations; along the AML 330 radial with a Tower-stated altitude of 1400 ft. MSL or below.The simultaneous events of directing 1400 ft. and below; calling out traffic requiring an external scan; and a turn toward rising terrain; dictated an ultimate loss of adequate terrain clearance. The loss of clearance likely caused disturbance to persons on the ground; and would have caused significant navigational hazard during visual approach at night.The nature of the virtual Tower permitting; JYO Tower needs a Class D airspace and Class B exclusion up to 2500 ft. AGL to provide adequate control volume to contain departures from the west. Further guidance of airspace constraints; such as 'Turn downwind; maintain at or below 2500 ft. outside AML 12 DME or visual checkpoint would also assist pilots who are scanning outside the airplane instead of consulting charts inside the cockpit.